[cfe-dev] About BlockDecl

Ted Kremenek kremenek at apple.com
Thu Dec 3 23:07:17 PST 2009

On Dec 3, 2009, at 6:49 PM, Zhongxing Xu wrote:

> Hi,
> I have two questions about BlockDecl.
> 1. Is it possible to distinguish the following decls in AST:
> ^ void (void) {  };
> ^ (void) {};
> ^{};

Hmm.  It doesn't look like BlockDecl currently doesn't store enough source information to distinguish these cases, although I may be wrong.

> 2. When is BlockDeclRefExpr used?

It's used within a block to refer to a "captured" variable from outside the block.  From SemaExpr.cpp:

  // If the identifier reference is inside a block, and it refers to a value
  // that is outside the block, create a BlockDeclRefExpr instead of a
  // DeclRefExpr.  This ensures the value is treated as a copy-in snapshot when
  // the block is formed.

For example:

void foo(void) {
  int x = 0;
  ^{ x + 1; }();

The 'x' within the block would be BlockDeclRefExpr, as it refers to a "captured" variable outside the block.  By default, captured variables are literally copied when the block is created, but if a __block annotation precedes the declaration, the variable is captured by reference.  e.g.:

void bar(void) {
  __block int x = 0;
 ^{ x = x + 1; }();

Here 'x' in bar is literally modified by the block.

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